Method and apparatus for controlling a grinding mill



J. H. wlLsoN 3,145,935

METHOD AND APPARATUS FOR CONTROLLING A GRINDING MILL Aug. 25, 1964 Filed Dec. 28. 1961 i /NVENTOR JAMES H. W/LSON 5 wa Alla/'nay l Q. Il

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United States Patent O 3,145,935 METHOD AND APPARATUS FR CONTROLLING A GRINDING M[LL James H. Wilson, Franklin Township, Westmoreland County, Pa., assignor to United States Steel Corporation, a corporation of New Jersey Filed Dec. 28, 1961, Ser. No. 162,746 9 Claims. (Cl. 241-24) This invention relates to an improved method and apparatus for controlling feed of solids and water to a closed-circuit Wet grinding system.

A conventional closed-circuit wet-grinding system includes a grinding mill (for example a rod or ball mill) and a hydraulic classifier. A pulp composed of partially ground solids and Water continuously goes from the mill to the classifier, along with additional water. The classier separates solids in the pulp into an overflow product which leaves the system, and an underflow product which returns to the mill for further grinding of the solids therein, along with new solids and water.

For the mill to operate efliciently, both the quantity of solids inside the mill and the solids-to-water ratio must be maintained at predetermined levels. Conventionally the system also includes automatic control means for raising or lowering the rate at which new solids and water feed to the mill as the load inside the mill changes. Control means presently known gauge the noise level of the mill as it turns, or measure the power required to drive the mill, or observe some characteristic of the pulp as it discharges from the mill and thereby determine whether the mill is loaded properly. However, they have a disadvantage that they cannot detect improper loading and make a correction until a change actually takes place inside the mill. For example, solids more difficult to grind may reach the mill at any time, whereupon there is an immediate increase in the relative proportion of underilow particles in the pulp discharging from the mill. The classifier commences to return a greater quantity of solids to the mill for further grinding, while the quantity of new solids reaching the mill remains unchanged. Soon the greater quantity of returning solids overloads the mill, and the control acts to lower the feed rate of new solids, but not before the mill already is overloaded and operating inefliciently. Until the undesirable loading actually exists, there is no change discernible in the noise level, power requirement or pulp characteristic.

An object of the present invention is to provide an improved automatic control method and apparatus in which the need for correction in the feed rate of either the solids or water or preferably both is ydetected and the correction made before there is any change in the load inside the mill itself.

A further object is to provide an improved automatic control method and apparatus in which prospective changes in the mill load are detected by observation of the classifier underfiow product before it returns to the mill and corrections are made before the mill is improperly loaded,

A more specific object is to provide an improved automatic control method and apparatus in which continuous quantitative determinations are made of the solids and water components of the classifier underflow product, computations are made from these determinations of the quantities of newly fed components needed to maintain the desired mill load and water ratio, and the feed rates of new solids and water are adjusted accordingly before there is any change in the load inside the mill.

In the drawing:

The single figure is a diagrammatic perspective View of a closed-circuit wet-grinding system equipped with my improved control apparatus.

3,145,935 Patented Aug. 25, 1964 p ICC The drawing shows a grinding system which includes a rod or ball bill 10 and a hydraulic classifier 12. New solids feed continuously from a bin 13 to a conveyor belt 14 and thence into the mill. The bin is equipped with a table feeder 15, which has a variable speed drive motor 16. The quantity of solids feeding from the bin to the belt of course varies with the speed of this motor. Water enters the mill through a pipe 17 which contains an adjustable control valve 18. The belt and mill have any suitable drive mechanisms, not shown. A pulp composed of partially ground solids and water discharges continuously from the mill into a sump 19 where more water is added through a pipe 20. A pump 21 delivers pulp and additional water from sump 19 through a pipe 22 to the classifier 12. A pipe 23 carries an overow product of solids and water from the classifier for dewatering and further treatment outside the system. A pipe 24 returns an underflow product composed of solids and water to mill 10 for further grinding of the solids component thereof. The structure thus far described is conventional; hence I have not shown or described it in greater detail.

In accordance with the present invention, I install in pipe 24 a ow meter 28 and a percent-solids meter 29. The flow meter continuously transmits a signal a representative of the volume of underflow product (water and solids) leaving the bottom of the classifier 12. The percent-solids meter continuously transmits a signal b representative of the percent solids in the underflow product. Both these signals got to an analog computer multiplier 30 which continuously computes a signal c representative of the absolute quantity of the solids component of the underflow product and a signal d repersentative of the absolute quantity of the water component thereof (assuming both components are controlled). Signals c and d go respectively to summing amplifiers 31 and 32 which automatically compute the quantities of new solids and water respectively to be fed to the grinding mill 10, as hereinafter explained.

I connect a set point indicator 33 to the summing amplifier 31 and manually adjust this indicator to a setting representative of the total quantity of solids I desire in the grinding mill. The indicator continuously transmits to the summing amplifier a signal e proportionate to its setting. The summing amplifier continuously computes the difference between signal c (returning solids) and signal e (desired solids). This difference of course represents the quantity of new solids which the table feeder 15 should feed to belt 14 and thence to the grinding mill iti to maintain the total quantity of solids entering the mill at the desired level. A signal f reprentative of this difference goes to a speed controller 34, which is operatively connected to the table feeder motor 16. Belt 14 runs over a belt scale 35 which furnishes a pneumatic pressure signal g proportionate to the weight of solids the belt carries. This pressure signal goes to a transducer 36 which continuously transmits to the speed controller 34 a signal h representative of the actual quantity of new solids feeding to the mill. Belt scales also are known which produce electric signals directly. I can of course use a scale of that type and thus eliminate the transducer. Signals f and h should balance; whenever one is greater than the other, the speed controller acts to change the speed of the table feeder 15 to bring them into balance. Thus the feeder always delivers the quantity of new solids needed to maintain a constant load in the grinding mill, despite Variations in the quantity of returning solids.

I connect another set point indicator 40 to the summing amplifier 32 and manually adjust this indicator to a setting proportionate to the solids-to-water ratio I desire in the grinding mill. The indicator continuously transmits to the summing amplifier a signal i proportionate to its setting. The summing amplifier continuously computes the difference between signal a.' (returning water) and signal i (desired water). This difference of course represents the quantity of water which I should introduce to the mill via pipe 17. A signal j representative of this difierence goes to a valve-actuator controller 4i which is operatively connected to an actuator i2 for valve 18. I install a flow meter 43 in pipe 17 to measure the actual quantity of water flowing through this pipe into the mill. A signal k representative of this quantity goes to controller 41. Signals j and k should balance; whenever one is greater than the other, the controller acts to adjust valve 1S to bring them into balance. Thus valve I8 always controls the flow of water into the grinding mill to maintain a desired solids-to-water ratio, despite variations in Aboth the quantity of water returning to the mill and the quantity of new solids feeding thereto.

The individual devices I use in my control apparatus are known devices commercially available. Conveniently the flow meters 2S and 43 can be of the magnetic type. Reference can be made to Sturgeon Patent No. 2,800,016 or Sturgeon et al. Patent No. 2,867,119 for showings of typical magnetic flow meters suitable for my purpose. Conveniently the percent-solids meter 29 can be an Ohmart cell arranged to measure the density of the underflow product from the classifier. Reference can be made to Ohmart Patent No. 2,763,790 and to other disclosures mentioned therein for explanations of principles involved in an Ohmart cell and of the way such cells can be used to measure density. This same patent also explains how an electric signal from an Ohmart cell representative of density and another electric signal representative of volume can go to an analog computer which multiples the two signals to obtain signals representative of absolute quantities. Reference can be made to a printed publication entitled Catalog and Manual on GAP/ R High-Speed All-Electronic Analog Computers for Research and Design, published by George A. Philbrick Researches, Inc., 230 Congress St., Boston, Mass. (copyright 1951) for showings of an analog computer (page 18) and a summing amplifier (page 5) suitable for my purpose. Reference can be made to Frazel Patent No. 2,664,286 and Carlson Patent No. 2,059,549 respectively for showings of a belt scale and transducer suitable for my purpose. Since these devices per se are not of my invention and published showings can be found elsewhere, I have not shown nor described them in detail.

According to my control method, I continuously measure the rate of flow of the underflow product from classifier I2 and the percent solids therein. I continuously multiply these factors to determine the quantites of one or preferably both the solids and water components of the underflow product which will return to the grinding mill before they actually reach the mill. When I control both the solids and water, I set my indicators 33 and 40 in accordance with the quantity of solids and the solids-to- Water ratio I desire to maintain. I then automatically adjust the quantities of new solids and water as the returns vary to maintain the totals at the values I set on the indicator. I perform all these adjustments before there is any actual change in the mill load, and thus avoid any need for improperly loading the mill as a prerequisite for making corrections.

While I have shown and described only a single embodiment of my invention, it is apparent that modifications may arise. Therefore, I do not Wish to be limited to the disclosure set forth but only by the scope of the appended claims.

I claim:

l. In a closed-circuit wet-grinding process in which new solids and water components feed to a grinding mill, a pulp composed of solids and water goes from the mill to a hydraulic classifier, an overflow product is recovered from the classifier, and an underflow product composed of solids and water returns from the classifier to the mill for further grinding of the solids component thereof, a control method comprising determining individually the quantities of the returning solids and water components of the underflow product before it reaches the mill, setting the total quantity of solids and the solids-to-water ratio desired in the mill, developing a signal proportionate to the difference between the quantity of returning solids and the setting for total solids, developing another signal proportionate to the difference between the quantity of returning water and the desired quantity determined by the ratio setting, and adjusting the quantities of new solids and water components feeding to the mill in accordance with said signals to maintain both the total quantity of solids entering the mill and the solids-to-water ratio in the mill substantially constant as long as the settings remain unchanged before actual changes take place in the load inside the mill.

2. In a closed-circuit wet-grinding system which includes a grinding mill, an adjustable feeder for feeding new solids to said mill, an adjustable source for introducing water to said mill, a hydraulic classifier, means for transferring a pulp composed of solids and water from said mill to said classifier, means for recovering an overflow product from said classifier, and means for returning an underflow product composed of solids and water from said classifier to said mill for further grinding of the solids component thereof, the combination with said System of a control apparatus comprising means for determining individually the quantities of returning solids and water components of the underflow product before it reaches said mill, means operatively connected with said feeder for adjusting it in accordance with the quantity of returning solids thus determined to maintain the total quantity of solids entering said mill at a predetermined level, and means operatively connected with said water source for adjusting it in accordance with the quantity of returning water thus determined to maintain the solidsto-water ratio in said mill at a predetermined level which is independent of the feed rate of new solids, the means for adjusting said feeder and said water source being operable before actual changes take place in the load inside the mill.

3. In a closed-circuit Wet-grinding system which includes a grinding mill, an adjustable feeder for feeding new solids to said mill, an adjustable source for introducing water to said mill, a hydraulic classifier, means for transferring a pulp composed of solids and water from said mill to said classifier, means for recovering an overflow product from said classifier, and means for returning an underflow product composed of solids and water from said classier to said mill for further grinding of the solids component thereof, the combination with said system of a control apparatus comprising means for determining the quantity of returning water in the underflow product before it reaches said mill, and means operatively connected with said water source for adjusting it in accordance with the quantity of returning water thus determined to maintain the solids-to-water ratio in said mill at a predetermined level before actual changes take place in the load inside the mill.

4. In a closed-circuit wet-grinding system which includes a grinding mill, an adjustable feeder for feeding new solids to said mill, an adjustable source for introducing water to said mill, a hydraulic classifier, means for transferring a pulp composed of solids and water from said mill to said classifier, means for recovering an overflow product from said classifier, and means for returning an underflow product composed of solids and water from said classifier to said mill for further grinding of the solids component thereof, the combination with said system of a control apparatus comprising means for determining the quantities of returning solids and water components of the underflow product before it reaches said mill, means for setting the total quantity of solids and the solids-to-water ratio desired in the mill, means operatively connected with said solids-determining means and with said setting means for developin a signal proportionate to the difference between the total quantity of solids desired and the quantity of returning solids, means operatively connected with said water-determining means and with said setting means for developing a signal proportionate to `the difference between the quantity of water needed to make the desired ratio and the quantity of returning water, means operatively connected with said feeder for adjusting it in accordance with said first named signal, and means operatively connected with said water source for adjusting it in accordance with said second named signal to maintain the total quantities of solids and water entering the mill at the settings before actual changes take place in the load inside the mill.

5. In a closed-circuit wet-grinding process in which new solids and water components feed to a grinding mill, a pulp composed of solids and water goes from the mill to a hydraulic classifier, an overflow product is recovered from the classifier, and an underflow product composed of solids and water returns from the classifier to the mill for further grinding of the solids component thereof, a control method comprising determining individually the quantities of solids and water in the underflow product before it reaches the mill, adjusting the quantity of new solids in accordance with the quantity of solids returning in the underflow product to maintain the total quantity of solids entering the mill at a predetermined level, adjusting the quantity of new water in accordance with the quantity of water returning in the underflow product to maintain the solids-towater ratio in the mill at a predetermined level which is independent of the feed rate of new solids, both adjustments being made before actual changes take place in the load inside the mill.

6. In a closed-circuit wet-grinding process in which new solids and water components feed to a grinding mill, a pulp composed of solids and water goes from the mill to a hydraulic classifier, an overflow product is recovered from the classifier, and an underflow product composed of solids and water returns from the classifier to the mill for further grinding of the solids component thereof, a control method comprising determining both the total quantity of underflow product and the proportion of solids therein before the underflow product reaches the mill, computing from these determinations the individual quantities of solids and water in the underflow product, adjusting the quantity of new solids in accordance with the quantity of `solids returning in the underflow product to maintain the total quantity of solids entering the mill at a predetermined level, adjusting the quantity of new water in accordance with the quantity of water returning in the underflow product to maintain the solids-to-water ratio in the mill at a predetermined level which is independent of the feed rate of new solids, both adjustments being made before actual changes take place in the load inside the mill.

7. In a closed-circuit wet-grinding process in which new solids and Water components feed to a grinding mill, a pulp composed of solids and water goes from the mill to a hydraulic classifier, an overow product is recovered from the classifier, and an underflow product composed of solids and water returns from the classifier to the mill for further grinding of the solids component thereof, a control method comprising determining individually the quantities of `solids and water in the underflow product before it reaches the mill, setting the total quantity of solids desired in the mill, adjusting the quantity of new solids entering the mill in accordance with the quantity of solids returning in the underflow product to maintain the total quantity of solids substantially constant as long as the setting remains unchanged, setting the solidsto-water ratio desired in the mill, and adjusting the quantity of new water in accordance with the quantity of water returning in the underflow product to maintain the ratio substantially constant as long as the latter setting remains unchanged, both adjustments being made before actual changes take place in 4the load inside the mill.

8. In a closed-circuit wet-grinding process in which new solids and water components feed to a grinding mill, a pulp composed of solids and water goes from the mill to a hydraulic classifier, an overflow product is recovered from the classifier, and an underflow product composed of solids and water returns from the classifier to the mill for further grinding of the solids component thereof, a control method comprising determining both the total quantities of solids and water in the underflow product, solids therein before the underflow product reaches the mill, computing from these determinations the individual quantites of solids and water in the underflow product, setting the total quantity of solids desired in the mill, adjusting the quantity of new solids entering the mill in accordance with the quantity of solids returning in the underflow product to maintain the total quantity of solids substantially constant as long as the setting remains unchanged, setting the solids-to-water ratio desired in the mill, and adjusting the quantity of new water in accordance with the quantity of water returning in the underflow product to maintain the ratio substantially constant as long as the latter setting remains unchanged, both adjustments being made before actual changes take place in the load inside the mill.

9. In a closed-circuit wet-grinding system which includes a grinding mill, an adjustable feeder for feeding new solids to said mill, an adjustable source for introducing water to said mill, a hydraulic classifier, means for transferring a pulp composed of solids and water from said mill to said classifier, means for recovering an overflow product from said classifier, and means for returning an underflow product composed of solids and water from said classifier to said mill for further grinding of the solids component thereof, the combination with said system of a control apparatus comprising means for determining the total quantity of underflow product and the proportion of solids therein before the underflow product reaches said mill, means operatively connected with said determining means for computing from these determinations the individual quantities of solids and water in the underflow product, means operatively connected with said computing means and with said feeder for adjusting the latter in `accordance with the quantity o-f solids returning in the underflow product to maintain the total quantity of solids entering said mill at a predetermined level, and means operatively connected with said computing means and with said water source for adjusting the latter in accordance with the quantity of water in the underflow product to maintain the solids-to- Water ratio in said mill at a predetermined level which is independent of the feed rate of new solids, both said adjusting means being operable before actual changes take place in the load inside the mill.

References Cited in the file of this patent UNITED STATES PATENTS UNITED STATES PATENT o-EETCE CERTIFICATE OF CORRECTION Patent No.l 3, 145,935 August 25's] 1964 James H, Wilson It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as ooz'reoted below.

Column 6g line i7Y for "quantities of solids and water in the underflow product," read quantity of underflow product and the proportion of Signed and sealed this 8th day of December 19MLI (SEAL) Attest:

ERNEST W.. SWIDER EDWARD J. BRENNER Azitesting Officer Commissioner of Patents 

1. IN A CLOSED-CIRCUIT WET-GRINDING PROCESS IN WHICH NEW SOLIDS AND WATER COMPONENTS FEED TO A GRINDING MILL, A PULP COMPOSED OF SOLIDS AND WATER GOES FROM THE MILL TO A HYDRAULIC CLASSIFIER, AN OVERFLOW PRODUCT IS RECOVERED FROM THE CLASSIFIER, AND AN UNDERFLOW PRODUCT COMPOSED OF SOLIDS AND WATER RETURNS FROM THE CLASSIFIER TO THE MILL FOR FURTHER GRINDING OF THE SOLIDS COMPONENT THEREOF, A CONTROL METHOD COMPRISING DETERMINING INDIVIDUALLY THE QUANTITIES OF THE RETURNING SOLIDS AND WATER COMPONENTS OF THE UNDERFLOW PRODUCT BEFORE IT REACHES THE MILL, SETTING THE TOTAL QUANTITY OF SOLIDS AND THE SOLIDS-TO-WATER RATIO DESIRED IN THE MILL, DEVELOPING A SIGNAL PROPORTIONATE TO THE DIFFERENCE BETWEEN THE QUANTITY OF RETURNING SOLIDS AND THE SETTING FOR TOTAL SOLIDS, DEVELOPING ANOTHER SIGNAL PROPORTIONATE TO THE DIFFERENCE BETWEEN THE QUANTITY OF RETURNING WATER AND THE DESIRED QUANTITY DETERMINED BY THE RATIO SETTING, AND ADJUSTING THE QUANTITIES OF NEW SOLIDS AND WATER COMPONENTS FEEDING TO THE MILL IN ACCORDANCE WITH SAID SIGNALS TO MAINTAIN BOTH THE TOTAL QUANTITY OF SOLIDS ENTERING THE MILL AND THE SOLIDS-TO-WATER RATIO IN THE MILL SUBSTANTIALLY CONSTANT AS LONG AS THE SETTINGS REMAIN UNCHANGED BEFORE ACTUAL CHANGES TAKE PLACE IN THE LOAD INSIDE THE MILL. 